Due to their special properties, measuring elements comprising piezoelectrics or piezoelectric bodies are used in a number of sensor structures with measuring elements.
US Patent Application Publication No. 2012-0216175 A1, which is hereby incorporated herein by this reference for all purposes, discloses a sensor structure that consists of a plurality of measuring elements comprising multiple bodies of piezoelectric materials. The measuring elements can be used for measuring pressures and/or forces. The sensor structure has a compact design, wherein the measuring elements of piezoelectric material are arranged upright in a housing in a prestressed fashion and spaced apart from one another in order to utilize the piezoelectric effect for a dynamic force or pressure measurement. Forces are transmitted to the end faces of the measuring elements via a diaphragm. Measuring signals for determining the respective force or pressure are recorded in dependence on the strain of the measuring elements. At least one additional measuring element is provided for recording static force or pressure values, wherein this additional measuring element utilizes the inverse piezoelectric effect and is operated in the form of a thickness shear oscillator. This thickness shear oscillator is operated in the form of a piezoelectric resonator and incited to oscillate accordingly with an electronic excitation signal by means of electrodes applied to the lateral faces of the measuring element. The resonant frequency of the thickness shear oscillator changes in dependence on the force or pressure acting thereupon, wherein this can be used for respectively determining the static force or the pressure.
Numerous components are required for assembling such a sensor structure and have to be exactly oriented and mounted in the housing, as well as electrically wired with great care, such that the different measuring elements are not disturbed by the incited high-frequency oscillations of the other measuring elements and the excitation voltage of the electronic system used. As a result of the constructive design, the measuring elements are locally spaced apart such that the placement and the contacting of the measuring elements in the housing are in fact simplified, but force and/or pressure measurements consequently take place at locally different points and therefore lead to measuring inaccuracies.
AT503558 describes a different approach for carrying out a force or pressure measurement as close as possible to a location in a reproducible fashion and for improving the measuring accuracy. A measuring element comprising a body made of a piezoelectric material is provided with an additional second measuring element consisting of a piezoelectric measuring lamina with a so-called SAW (Surface Acoustic Wave) structure. The measuring element conventionally delivers the predominant dynamic pressure by utilizing the piezoelectric effect. An additionally compacted sensor structure is achieved because the SAW structure is arranged on the body made of piezoelectric material such that it directly contacts the measuring element. The measuring element features surface electrodes that likewise have to be mounted on the measuring element and tuned to the SAW structure before the surface electrodes and the SAW structure can be connected to an electronic evaluation and excitation system.
In the measuring mode, the electronic evaluation and excitation system generates surface waves in the form of acoustic waves along the SAW structure, wherein the transit times of these acoustic waves change in dependence on the force acting upon the measuring element and the SAW structure. The absolute pressure can be determined from the change of the transit times by means of the SAW structure.
Although fixing the SAW structure on the measuring element leads to a multipart sensor structure that respectively allows a pressure or force measurement at nearly the same location, inhomogeneous force distributions in the measuring element and in the SAW structure occur as a result. In the mounting of the SAW structure on the measuring element, as well as in the arrangement of the surface electrodes on the measuring element, it must be ensured that neither the two independent piezoelectric elements, nor the measuring signals originating from these two piezoelectric elements, disturb one another. In order to realize the sensor structure according to AT 503558, the measuring element has to be exactly connected to the piezoelectric measuring lamina with the SAW structure in an elaborate and therefore cost-intensive manufacturing step.